Empty arrays are turned into nil

[pebrc]

When parsing empty arrays from YAML to config objects and unpacking into an untyped representation like map[string]interface{} arrays in the original source are turned into nil. Marshalling back to YAML this turns into null and is thereby not round-trippeable through ucfg:

Consider:

node.roles: []

input, err := yaml.Unmarshal(`node.roles: []`)
...
c, err := NewConfig([]byte(input))
...
var output map[string]interface{}
err = c.Unpack(output)
...
yaml.Marshal(output)

leaves us with:

node.roles: null

We have been hit in the ECK operator a couple of times by this issue:

• Kibana Fleet config uses empty arrays to turn off monitoring: Kibana YAML settings with empty array values get parsed as null cloud-on-k8s#4853
• Elasticsearch uses empty arrays to express the coordinating node role Coordinator nodes cannot be defined using node.roles cloud-on-k8s#3718

We are using untyped map[string]interface{} target data structures in the operator for application configuration because it is difficult for us to model each Elastic stack application's configuration completely and for all versions of the Elastic stack that we need to support.